Recently, with increasing consciousness about environmental problems, solar batteries have attracted attention as clean energy sources. A solar cell module for forming a solar battery includes solar cell elements, which play a role to convert light energy, such as sunlight, to electric energy.
Solar cell elements are often made using a monocrystalline silicon substrate or a polycrystalline silicon substrate. Thus, solar cell elements are vulnerable to mechanical impact, and a solar cell module placed outdoors needs to be protected from rain, etc. Since electric power generated from a piece of solar cell element is small, a plurality of solar cell elements must be connected in series and parallel so that a practical level of electric power can be output. Thus, a solar cell module is commonly made by connecting a plurality of solar cell elements and sealing them with a transparent substrate and a sealing material. In general, a solar cell module is produced by sequentially stacking members including a transparent front substrate, a sealing material, solar cell elements, another sealing material, and a rear surface-protecting sheet, etc., and subjecting them to a lamination process or the like in which they are subjected to vacuum aspiration and thermocompression bonding.
In view of processability, workability, manufacturing cost, and other points, ethylene-vinyl acetate copolymer (EVA) resin is most commonly used as a sealing material for solar cell modules. Unfortunately, EVA resin tends to gradually decompose over long-term use and may deteriorate in a solar cell module to cause a reduction in strength or to produce acetic acid gas, which will affect solar cell elements. Thus, it is proposed that in place of EVA resin, polyolefin-based resin such as polyethylene should be used to form a sealing material for use in a solar cell module.
For example, Patent Document 1 discloses a sealing material layer containing a crosslinking agent and an ethylene-α-olefin copolymer obtained by polymerization using a single site catalyst. Patent Document 2 discloses a transparent, flame-retardant layer containing a crosslinking agent and a resin obtained by graft polymerization of polyethylene and an ethylenic unsaturated silane compound.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2000-91611
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2009-10277